1. Field of the Invention
The present invention relates to a sheet processing apparatus that applies a post process to a sheet.
2. Description of the Related Art
There are known conventional sheet processing apparatuses each of which applies a post process like a punching process to a sheet on which an image has been formed. Such a sheet processing apparatus detects misalignment of a sheet in a sheet width direction that is perpendicular to a sheet conveyance direction (referred to as “lateral misalignment”, hereafter), and corrects the lateral misalignment in order to increase accuracy of a hole position of a punching process.
There is a known method that moves an optical sensor in the sheet width direction, and obtains an amount of lateral misalignment of a conveyed sheet based on the timing at which the optical sensor detects the lateral end portion (side end) of the sheet.
However, when the sheet is skew, an error difference may occur between the amount of the lateral misalignment at the detected side end and the amount of the lateral misalignment at a portion where a punch hole will be formed (for example, a sheet rear end). Accordingly, it is necessary to obtain correctly the amount of the lateral misalignment in the sheet rear end in consideration of the skew amount of the sheet in order to increase accuracy of the punching hole position.
Incidentally, there is a known apparatus that efficiently detects a sheet side end by moving a sensor unit that consists of a plurality of photo sensors in a sheet width direction during a sheet conveyance (see U.S. Pat. No. 8,066,279).
As shown in FIG. 13, this kind of apparatus detects a sheet side end with a plurality of sensors 1104a through 1104c arranged on a lateral position sensor unit 1105 in a sheet width direction, and can calculate a skew amount and a lateral misalignment. That is, the lateral position sensor unit 1105 is moved in the sheet width direction during a sheet conveyance, and the lateral position sensor 1104a detects a side end at the first time. Then, the lateral position sensor 1104b detects the side end at the second time after the lateral position sensor unit 1105 is further moved. Then, the skew amount of the sheet etc. are calculated based on the two detection results.
Like the above-mentioned conventional apparatus, when calculating the skew amount of a sheet using a plurality of sensors, proportional calculation is performed using two sheet-side-end detection results. Accordingly, the sheet conveyance distance from the first detection to the second detection participates in calculation.
However, when the second detection timing is too early, the distance from the position of the second detection to the sheet rear end in the sheet conveyance direction becomes long, and the sheet conveyance distance from the first detection to the second detection becomes short. In that case, an error becomes large in the skew amount calculated using the proportional calculation.
On the other hand, the sensor that detects at the second time needs to reach the sheet side end before the sheet rear end passes the sensor position in order to detect the side end certainly at the second time. In order to correspond to all sheet sizes, the interval between the sensors and the moving velocity of the sensor unit have to be set with a margin so that the side end can be detected at the second time even in a sheet with the shortest sheet length among assumed sheets. However, such a uniform setting must shorten the sheet conveyance distance between the first detection and the second detection.
When the calculation accuracy of the skew amount decreases, an accuracy of a process using the skew amount (a lateral misalignment correction, for example) decreases, which decreases accuracies of a hole position on a sheet etc.